Introduced, naturalised or invasive in East Africa

Habitat

Azolla filiculoides is a plant of slow flowing streams and rivers, ponds and lakes in warm, tropical climates where the minimum water temperature remains above 0°C throughout the year.

Description

Azolla filiculoides is a small, free floating freshwater fern, green to reddish-brown or purplish orange or red at the edges, branching freely, and breaking into smaller sections as it grows. The adult plant is approximately 25-35 mm long, with the length of the individual frond ("leaves") being approximately 1-1,5 mm. Plants can change colour from green to brown and red as a result of changes in sunlight intensity (and shade) as well as ambient temperature.

Reproduction and dispersal

Azolla filiculoides is able to undergo rapid vegetative reproduction throughout the year by the elongation and fragmentation of the small fronds. Under ideal conditions an infestation can double in area every 4-5 days. At such growth rates it is capable of completely covering pond and lake surfaces in a matter of weeks or months. Under favourable environmental conditions, A. filiculoidesfern undergoes sexual reproduction through spores (Henderson and Cilliers, 2002). It can also be dispersed on the feet and feathers of water birds and on/by mammals such as hippos and otters. It has been spread by the garden and aquaria trade and can find its way into water bodies through discarded garden or pond waste and flood events.

Similar species

Azolla filiculoides can be confused with the native azollas - Azolla nilotica and Azolla pinnata. A. filiculoides differs from A. nilotica in having 1-3 roots per node whereas A. nilotica has numerous (5 or more) roots per node. A. nilotica has aerial parts that extend upwards from the floating parts and it is much larger than A. filiculoides. A. pinnata, has 2-3 roots per node, but the plant is more noticeably pinnate, the leaves are smaller and more acute at the apex and the plant is never reddish.

Economic and other uses

The species has been introduced to many regions of the Old World, grown for its nitrogen-fixing ability which can be utilised to enhance the growth rate of crops grown in water like paddy rice, or by removal from lakes for use as green manure. It is also used as an ornamental plant in ponds. However, these uses cannot compensate for this plant's overall negative impacts.

Environmental and other impacts

Azolla filiculoides forms dense mats and outcompetes native plant species. These infestations can reduce light levels below the mats and cause die off of water plants and algae and reduce water oxygenation levels with serious impacts on fish and other fauna. Not only can very little survive under such conditions, but the quality of drinking water is reduced, caused by bad odours, colour and turbidity. A. filiculoides can block canals, drains and overflows and may lead to an increased risk of flooding. . It is can also affect irrigations systems - both by blocking their water supply and by reducing water quality. The economic impact of A. filiculoides in South Africa was calculated by McConnachie et al. (2003). Among those water-uses most seriously affected were farming (71%), recreational (24%), and municipal (5%). On average, A. filiculoides was found to cause on-site damages of US$589 per hectare per year.

A. filiculoides is on the Federal Noxious Weeds List in the USA and has been listed as a noxious weed in South Africa (prohibited plants that must be controlled. They serve no economic purpose and possess characteristics that are harmful to humans, animals or the environment).

Management

The precise management measures adopted for any plant invasion will depend upon factors such as the terrain, the cost and availability of labour, the severity of the infestation and the presence of other invasive species. Some components of an integrated management approach are introduced below.

The best form of invasive species management is prevention. If prevention is no longer possible, it is best to treat the weed infestations when they are small to prevent them from establishing (early detection and rapid response). Consistent follow-up work is required for sustainable management.

Azolla filiculoides should not be released in the wild. Dispose of garden and pond waste responsibly. Check vehicles, boats, fishing gear, equipment and clothing for plant fragments when leaving infested sites and clean if necessary.

Physical control is difficult as A. filiculoides reproduces by fragmentation and, in rivers, these fragments float downstream and are capable of regrowth into cleared areas within days.

Biological control using the weevil Stenopelmus rufinasus can be extremely effective in controlling infestations as both adults and larvae feed on the fronds (Hussner 2006).

Legislation

Not listed as a noxious weed by the state or governments in Kenya, Tanzania and Uganda.

Henderson, L. and Cilliers, C.J. 2002. Invasive aquatic plants-a guide to the identification of the most important and potentially dangerous invasive aquatic and wetland plants in South Africa. PPRI Handbook No. 16, Agricultural Research Council, Pretoria.www.arc.agric.za/uploads/images/0_SAPIA_NEWS_No._17.pdf.

Editors

Acknowledgments

This fact sheet is adapted from The Environmental Weeds of Australia by Sheldon Navie and Steve Adkins, Centre for Biological Information Technology, University of Queensland. We recognise the support from the National Museums of Kenya, Tropical Pesticides Research Institute (TPRI) - Tanzania and Makerere University, Uganda. This activity was undertaken as part of the BioNET-EAFRINET UVIMA Project (Taxonomy for Development in East Africa).